It is known that sintered ceramic structures can be prepared by firing aggregates of sinterable ceramic particles to above the sintering temperature. It is also known that firing can be effected by heating green ceramic bodies in a two stage sequence characterized in that the bodies are fired to a suitable first temperature below the sintering temperature under oxidizing flame conditions and thereafter fired to a higher temperature of not less than the sintering temperature under reducing flame conditions.
The prior art firing methods have not been entirely satisfactory for preparation of sintered ceramic structures from sinterable ceramics composited with organic binders which decompose at temperatures less than the sintering temperature of the ceramics. For example, attempted preparation of sintered ceramic structures from sinterable ceramics thus composited using prior art methods result with unacceptable frequency in ceramic structures which are fissured, distorted or otherwise defective.
The applicant herein has found that low quality ceramic structures result where flame propagation occurs on green bodies containing sinterable ceramics composited with combustible organic binders. Such flame propagation typically occurs in the presence of free oxygen during periods of firing at relatively low temperature prior to removal of the binder.
Evidence that at least one other worker recognized that severe problems in preparing ceramic structures from compositions including ceramic powder and thermoplastic binders are encountered in early stages of firing appears in U.S. Pat. No. Re. 28,195 to Sergeys. That patent discloses a method for preparing porous ceramic structures by shaping a ceramic filled particular polyolefin material containing a plasticizer, extracting the plasticizer, burning-off the polyolefin and firing the porous shaped ceramic structure. Thus, as taught in Example 1 thereof, a high molecular weight polyethylene of 0 Melt Index is burnt off by heating in an oxidizing atmosphere in the course of preparing porous ceramic structures. Although the method described in the aforesaid Sergeys patent effectively advanced the art, a need for even further improvement in order to minimize problems such as cracking and distortion has become apparent.